The invention relates to a method for preparing anhydrous dicalcium orthophosphate of predetermined and substantially uniform particle size for making calcium halophosphate phosphors for use in fluorescent lamps.
Phosphors comprising calcium halophosphates are used to convert ultraviolet radiations to visible light in fluorescent lamps. These phosphors may be represented by the general formula: EQU Ca.sub.5 (PO.sub.4).sub.3 (Cl, F): Mn, Sb
The major raw material used to prepare this class of phosphors is anhydrous dicalcium orthophosphate (CaHPO.sub.4), and it is this component which has the greatest influence on the finished phosphor particles, including characteristics such as size and shape. As is known in the art, a narrow particle size distribution of the phosphor is desirable to achieve maximum light output. In the usual commercial process, CaHPO.sub.4 is prepared by an essentially two-step process. The first step involves precipitating the dihydrate form of the calcium phosphate by mixing and reacting calcium chloride (or nitrate) solution with diammonium phosphate solution. The dihydrate is then recrystallized to anhydrous dicalcium orthophosphate of the desired purity, particle size and particle shape. The prior art methods for the recyrstallization or conversion of CaHPO.sub.4.2H.sub.2 O to CaHPO.sub.4 have very often yielded CaHPO.sub.4 materials containing many fine particles, coarse particles, agglomerates or combinations thereof.
A typical prior art method for making CaHPO.sub.4 involves heating an acidified (generally with H.sub.3 PO.sub.4) aqueous slurry of CaHPO.sub.4.2H.sub.2 O under vigorous agitation to the desired temperature range where the "recrystallization" occurs. In this temperature range (generally 70.degree.-95.degree. C.) the slurry gradually thickens and gels to a certain degree. The extent of the gelation is dependent on the nature of the CaHPO.sub.4.2H.sub.2 O used, the slurry concentration (solids content), the degree of acidification, the degree of agitation, the temperature, and the heating rate. After about 5-30 minutes of heating and agitation at the recrystallization temperatures, the gel begins to break down and when the slurry again appears homogeneous with no evidence of gel globules, the conversion to CaHPO.sub.4 is complete. The product formed generally consists of rather square, transparent, platelike crystals somewhat nonuniform in size, with many fines.
In U.S. Pat. No. 3,395,979 issued to Schafer, dated Aug. 6, 1968 is disclosed a continuous or semicontinuous process for the conversion of CaHPO.sub.4.2H.sub.2 O wherein the CaHPO.sub.4.2H.sub.2 O is raised to the desired temperature range and is allowed to form a gel column supported on a screen in a reaction vessel. As the conversion occurs, crystals of CaHPO.sub.4 fall through the screen and are separated from the gel. The process as disclosed involves continuously feeding dihydrate slurry and permitting converted CaHPO.sub.4 and residual liquid to be drained off. Continuous-type processes are subject to back mixing of converted and partially converted crystals resulting in agglomerations and particle size nonuniformity.